The use of container-processing machines, such as labelling machines for gluing and transferring labels onto the surface of containers, is very well known and widespread.
In these machines containers are typically carried by a carousel to come into contact with a labelling unit. The labelling unit comprises one or more so called “vacuum drums” by which a label material strip is received from a roll feeding system. The labelling system cuts the label off the label material strip at the appropriate length; glues the label by appropriate means (such as a gluing drum, spray and injector systems or the like); and finally transfers the glued label to a respective container. For performance of these operations, the label is, at different stages, retained on the outer surface of a vacuum drum by means of suction forces applied on the label. To this purpose, the surface of the vacuum drum comprises a plurality of orifices that are in fluid communication with a vacuum source.
On the one hand, the use of a thin film-like material as the label material entails a rather desirable reduction of production costs with respect to more conventional thicker alternatives. However, when a label material with reduced thickness and a low elastic modulus is used, a number of undesired issues are typically encountered, which are caused by an inherent weakness of such material.
It is a given fact that the labelling speed has become a key factor, especially in view of the high production requirements which are nowadays very common in canning and bottling plants. Speeds in excess of ten containers per second have regularly been achieved with certain labelling materials and label-handling processes.
However, while, in general, thicker materials with good elastic properties and low friction display a good resistance to stretching and are, consequently, relatively easy to handle at high said materials can withstand relatively great forces (i.e. a high degree of vacuum can be conveniently used for handling said materials throughout the process), when weaker label materials are run at high speeds and with high friction, a variety of undesirable events, such as splitting, stretching and misalignment of labels, often occur.
With roll-fed thin labels, when the labels are cut from the strip of labelling material, excessive tension on the label can cause the labels to split or tear instead of being cleanly cut.
Similarly, over-tensioning can cause thin labels to stretch as they are picked up by the vacuum drum. As the labels are transferred to a vacuum drum, excessive vacuum can cause the label segment to shift or snap, which is highly likely to cause a temporary crash of the machine, because the label shall get off its ideal path within the machine, glue may be spilled, and so forth. In other words, ripping of the label material typically results in an undesirable interruption of the overall labelling process.
Furthermore, some labelling materials may include coatings or have be subjected to other superficial treatments that result in higher coefficients of friction, which can also affect the labelling process. In particular, labels having a higher coefficient of friction tend to become over-tensioned more easily, which aggravates the problems associated with over-tensioning already described above.
Yet another problem encountered, when labels are supported by a vacuum drum during the labelling process, is that glue applicators for applying glue to the label segments can become jammed by labels, if insufficient vacuum is provided to prevent the labels from following the glue applicator.
As a result, a particularly thorough control of the operating conditions—mainly as concerns the degree of vacuum provided at the vacuum drum—is required in order for high speed labelling equipment to manage to effectively and reliably handle the weaker label materials referred to above, so that a fully satisfactory production performance may be attained in spite of lower elastic properties of the labelling material. To this purpose, in fact, a very specific degree of vacuum must be provided and maintained throughout operation of the labelling machine.
Also worth considering is the fact that labelling machines typically comprise a plurality of stations at which a vacuum drum or other vacuum-connected surface interacts with the labelling material. More often than not, for the sake of cost limitation and bulk reduction, these machines are generally equipped with a single vacuum source fluidically communicating with each and every vacuum drum through corresponding ducts, the flow along these ducts being alternately enabled and disabled, as a function of a number of processing parameters.
In particular, application (or suspension thereof) of vacuum to a portion of labelling material at a certain time shall depend on the position of the portion with respect to the overall labelling process (i.e. depending on which vacuum drum or surface in the whole of the machine is interacting with the portion of labelling material) and, even more particularly, on the angular position reached by the portion of labelling material upon rotation of a certain vacuum drum, so as to e.g. time the detachment of the label at a given station with the arrival of a container to be labelled at the very same station, and so forth.
To further complicate the picture, vacuum drums are often partitioned, i.e. a number of cavities are defined on their inside, each of which is independently connectable fluidically with the vacuum source, so that a single vacuum drum may be used for independently holding and handling several labels at a time, e.g. one for each separate internal cavity. In this case, the vacuum drum shall be designed so as to allow for alternate opening/interrupting of the fluidic connection of each internal cavity with the vacuum source, so that the supply of vacuum to each partition of the drum is timed with the intended interaction with the labels.
Start-up and shut-down represent particularly critical process phases, since, at those times, i.e. when the very first labels or the very last labels go round the system, a significant percentage of the holes in the vacuum drum are not covered by any labelling material, so a great amount of vacuum source power is potentially wasted.
U.S. Pat. No. 6,546,958 discloses a fixed vacuum plate assembly with a plurality of cavities for providing different levels of vacuum to a rotating vacuum drum in a container labelling apparatus, wherein thin and stretchable films can be swiftly and accurately applied with limited wastage and reduced occurrence of over-tensioning label material during the labelling process. In particular, the different cavities in the plate assembly are configured to be supplied with different levels of vacuum, each of which is suitable for a specific operation involving the label material (e.g. picking a label segment which has just been cut, gripping a label segment while an adhesive is applied to its surface, etc.). By improving the seal between the fixed valve plate and the rotating vacuum drum at the point of high vacuum, vacuum loss is reduced, thereby providing a more precise control of the vacuum at the point of adhesive application and eliminating the spread of high vacuum to adjacent ports that can negatively affect the label cutting and application by changing the vacuum level in these adjacent ports and chambers on a random basis.
However, this type of solution is based on a specific design of the vacuum drum and is therefore lacking in terms of versatility, because, for the whole labelling machine to adjust to different production requirements, the vacuum drums would have to be redesigned accordingly.
The need is therefore felt, in the art, for a container-processing machine of the type where suction is used for handling a sheet-like material—and even more particularly a weak label material—which machine enables selective and accurate control of the amount of vacuum supplied to each and every vacuum drum of a plurality of vacuum drums. In particular, a container-processing machine is needed that shall allow to calibrate, at any time, the amount of vacuum supplied to each vacuum drum in view of the current processing stage and of design parameters (e.g., in the case of a labelling machine, number of labels borne by each drum, operational speed, thickness, elastic properties and dimensions of the label material in use, etc.). Even more so, it is desirable that performance of the container-processing machine be adjustable to varying design parameters (e.g. when different vacuum drums with different characteristics are used with the same vacuum source and control system) in a straightforward and effective manner.